As an apparatus for removing sulfur oxides from combustion exhaust gas to prevent air pollution, a wet-type limestone-gypsum desulfurizing apparatus is being put to practical use widely. A system of this desulfurizing apparatus is shown in FIG. 9. An exhaust gas 1 from a boiler, etc., is introduced from a gas entrance 3 into an absorbing tower 4, and by the exhaust gas 1 coming into contact with droplets of an absorbing liquid sprayed from a plurality of spray nozzles 8a disposed in each of spray headers 8 installed in multiple stages in a gas flowing direction inside the absorbing tower 4, SOx in the exhaust gas 1 are absorbed, along with soot dust, hydrogen chloride (HCl), hydrogen fluoride (HF), and other acidic gases in the exhaust gas 1, at droplet surfaces. A mist entrained in the exhaust gas is eliminated by a mist eliminator 5 installed at an absorbing tower exit, and a clean exhaust gas 2 is emitted from a chimney via an exit flue 6 and upon being reheated if necessary.
A SOx concentration in the exhaust gas 1 flowing through the gas entrance 3 of the absorbing tower 4 in this process is measured by an entrance SOx meter 41. Limestone 16, which is a SOx absorbent, is kept in a limestone slurry tank 15, and the limestone slurry is supplied by a limestone slurry pump 17 to a reservoir 4a disposed at a lower portion inside the absorbing tower 4. An amount of the limestone slurry supplied to the absorbing tower 4 is adjusted by a limestone slurry flow control valve 18 according to a SOx absorption amount inside the absorbing tower 4.
The slurry-form absorbing liquid in the reservoir 4a inside the absorbing tower 4 is pressurized by an absorbing tower circulating pump 7 and supplied via a circulation piping 13 to the spray headers 8 disposed in multiple stages in the gas flow direction at an empty tower portion at an upper portion inside the absorbing tower 4. Each spray header 8 is provided with a plurality of spray nozzles 8a, and the absorbing liquid is sprayed from the spray nozzles 8a and put in gas-liquid contact with the exhaust gas. The SOx in the exhaust gas reacts with calcium compounds in the absorbing liquid and converted to calcium sulfite (including calcium bisulfite), which is an intermediate product, drops to the reservoir 4a of the absorbing tower 4, is oxidized to gypsum and thereby converted into a final product (gypsum) by air supplied by an oxidizing air blower 21 into the absorbing liquid of the absorbing tower 4.
By thus supplying air directly into the absorbing tower 4, the reaction of absorption of the SOx in the exhaust gas and the oxidization reaction of the calcium sulfite produced are made to proceed simultaneously to promote the overall reaction and improve desulfurization performance. The oxidizing air supplied to the absorbing tower 4 in this process is made into microscopic bubbles by an oxidizing agitator 26 that agitates the absorbing liquid inside the reservoir 4a to improve usage efficiency of the oxidizing air.
The absorbing liquid is thereafter extracted from the reservoir 4a by an extracting pump 9 in accordance with an amount of gypsum produced, and a portion thereof is fed to a pH meter tank 30 and a pH of the absorbing liquid is measured by a pH meter 31 installed in the pH meter tank 30. The remaining portion of the absorbing liquid is fed to a gypsum dehydration system 10 and recovered as powder gypsum 11.
Meanwhile, water 12, separated at the gypsum dehydration system 10, is reused inside the system as makeup water supplied to the limestone slurry tank 15, etc., and a portion thereof is extracted as wastewater 14 for preventing concentration of chlorine, etc., and fed to a wastewater treatment system 50. At the wastewater treatment system 50, a chemical process by addition of a chemical or treatment by an ion adsorption resin, etc., and a biological process by bacteria are performed to eliminate hazardous substances in the wastewater so that amounts of respective components in the waste water 14 fall below emission standards.
With the above-described conventional art, a portion of the droplets of the absorbing liquid sprayed from the spray nozzles 8a installed on the spray headers 8 drop to the reservoir 4a along a tower wall of the absorbing tower 4.
Because the absorbing liquid that drops along the absorbing tower wall portion absorbs hardly any SOx, an amount of the absorbing liquid sprayed from the spray nozzles 8a that is required to obtain a required desulfurization rate tends to increase.
A horizontal sectional view of the current absorbing tower 4 is shown in FIG. 8, and in the case of the cylindrical absorbing tower 4, a number of spray nozzles 8a lessens near the tower wall and a droplet density of the absorbing liquid at a portion near the tower wall of the absorbing tower 4 tends to be low. A downwardly directed absorbing liquid spraying angle of the spray nozzles 8a indicated by open circles in FIG. 8 is approximately 90 degrees.
When there is such a bias in spray droplet density of the absorbing liquid inside the absorbing tower 4, a large amount of the exhaust gas flows to a portion of low droplet density, preventing adequate gas-liquid contact, degrading exhaust gas SOx absorption performance at portions, and thus affecting the overall desulfurization performance.
As a countermeasure for the above, an invention, with which a ring is installed along an entire circumference of a tower wall portion of an absorbing tower to cause an absorbing liquid flowing along the tower wall to be rescattered toward a center portion of the absorbing tower, is proposed in Patent Document 1.
Meanwhile, an invention, with which a trough is disposed at an upper portion of a gas entrance of an absorbing tower to prevent drying up of solids in the absorbing liquid at the gas entrance, is proposed in Patent Document 2.
Patent Document 1: U.S. Pat. No. 6,550,751
Patent Document 2: Japanese Published Unexamined Patent Application No. 2001-327831